Clean up prev experiment.
Clean up prev experiment.

- Only need 4 (not 5) samples
- Document some of the math
- Use round of CTM to better match 'brute-force' version

Change-Id: Ibcc8e8eabc10158da8eecaba303385ed79218126
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/379847
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

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